1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and more particularly to a nonvolatile semiconductor memory device having a floating gate electrode.
2. Description of the Background Art
Recently, a flash memory, a kind of nonvolatile semiconductor memory devices, has come to be well known. The flash memory is expected to be a memory device of the next generation because it can be manufactured at lower cost than a dynamic random access memory (DRAM).
FIG. 17 is a sectional view showing a memory cell of such a conventional flash memory. Referring to FIG. 17, in the conventional flash memory, an N type drain diffusion layer 102 and an N type source diffusion layer 103 are formed spaced apart by a prescribed distance, with a channel region sandwiched therebetween, at the main surface of a P well 101 formed at the surface of a silicon substrate (Si substrate)(not shown). On the channel region, a floating gate electrode 105 of an N type polycrystalline silicon film (hereinafter referred to as a "polysilicon film") is formed with a tunnel oxide film 104 therebetween. On floating gate electrode 105, an interlayer insulating film 106 of an ONO film is formed. On interlayer insulating film 106, a control gate electrode 107 is formed of N type polysilicon. A side wall oxide film 108 is formed on both sides of floating gate electrode 105 and control gate electrode 107.
Source 103 is connected to a corresponding source line (not shown), and drain diffusion layer 102 is connected to a corresponding bit line (not shown). Floating gate electrode 105 is for accumulating charges representing information, and control gate electrode 107 is connected to a corresponding word line (not shown).
In operation, erasing or writing is carried out by injecting electrons into floating gate electrode 105 or by extracting electrons accumulated in floating gate electrode 105, with the FN tunnel phenomenon of tunnel oxide film 104 or the channel hot electron phenomenon. Thus, thresholding/binarization is realized by the state of electrons in floating gate electrode 105, and "0" or "1" is read out according to the state.
In the flash memory or EEPROM, amount of electrons accumulated in floating gate electrode 105 is thresholded, thus the state of the transistor is represented by binary values. Therefore, the memory function is implemented. By surrounding floating gate electrode 105 with insulating films (tunnel oxide film 104 and interlayer insulating film 106), electrons accumulated in floating gate electrode 105 can be kept for as long a period as more than ten years, and thus the nonvolatile memory is implemented. Specifically, by surrounding floating gate electrode 105 with high band barriers formed between insulating films (104, 106) having wide band gaps and floating gate electrode 105, electrons in floating gate electrode 105 are prevented from escaping outside. In short, electrons are confined in a well-type potential.
Normally, an SiO.sub.2 film and an ONO film are used as insulating films (104, 106) surrounding floating gate electrode 105. The ONO film is used as interlayer insulating film 106 between control gate electrode 107 and floating gate electrode 105.
As floating gate electrode 105, N type polysilicon is generally used which can accumulate electrons and achieve resistance that is low enough as a electrode. Polysilicon is a most generally used electrode material in the current ULSI process because of many reasons. One of them is that polysilicon provides good junction interface with the SiO.sub.2 film. Since the N type polysilicon which includes a large amount of N type impurity is N type, it has a lot of free electrons in a film and, as a result, it has a metal property of low resistance. In a floating type nonvolatile semiconductor memory, data is stored by injecting and extracting electrons into and out of floating gate electrode 105 to control the amount of electrons accumulated in floating gate electrode 105. As floating gate electrode 105, is typically formed of N type polysilcon which is a low resistance electrode having a large amount of free electrons. Since an N type polysilicon film is used as floating gate electrode 105 in a conventional flash memory, conduction band electrons in floating gate electrode 105 are extracted toward silicon substrate 101 through tunnel oxide film 104 by using FN tunnel current, as shown in FIG. 18.
If the floating gate type nonvolatile semiconductor memory is miniaturized in a similar manner as miniaturization of other ULSI devices, operating voltage would be lowered while insulating films such as tunnel oxide film 104 and interlayer insulating film 106 are made thinner. In ULSI device development, this is a general understanding known as a scaling rule of an MOS type transistor.
However, tunnel oxide film 104 and interlayer insulating film 106 which are extremely thin may cause part of electrons accumulated in floating gate electrode 105 to pass through tunnel oxide film 104 or interlayer insulating film 106 and to leak into the silicon substrate or control gate electrode 107, due to the FN tunnel phenomenon, the direct tunnel phenomenon or the tunnel phenomenon through a trap in an insulating film. FIG. 19 shows a band for illustrating a mechanism of leak current at the time of data retention (with no voltage applied) of a conventional write (or erase) state. If tunnel oxide film 104 is thin, conduction band electrons in floating gate electrode 105 are conventionally leaked due to the FN tunnel phenomenon as shown in FIG. 19, thus degrading the data retention characteristics. For simplicity, interlayer insulating film 106 is not an ONO film but an SiO.sub.2 film in FIG. 19.
When thin tunnel oxide film 104 is used in a floating gate type nonvolatile semiconductor memory such as the EEPROM, stress on tunnel oxide film 104 due to repetition of writing and erasing operations causes leak current if an electric field applied to tunnel oxide film 104 is low. This is disclosed, for example, in K. Naruke et. al., IEDM Tech. Dig., p424, 1988 (reference 1).
The low electric field leak current caused by such stress is called stress induced leak current. When this type of stress induced current is caused in the floating gate type nonvolatile semiconductor memory, electrons accumulated in floating gate electrode 105 are gradually lost, while memory data is retained, due to a small electric field applied to tunnel oxide film 104. Therefore, it is believed that such a thin tunnel oxide film 104 as would cause this type of stress induced leak current can not be applied for the floating gate type nonvolatile semiconductor memory. In short, the characteristics of the stress induced leak current limits thinness of tunnel oxide film 104 of the floating gate type nonvolatile semiconductor memory.
Naruke et. al. and R. Moazzami et. al., IEDM Tech. Dig., p139, 1992 (reference 2) report that the stress induced current is caused significantly when tunnel oxide film 104 is thinner than 10 nm.
When tunnel oxide film 104 can not be made thinner in the floating gate type nonvolatile semiconductor memory, the operating voltage can not be lowered. It is therefore difficult to reduce power consumption. For a floating gate type nonvolatile semiconductor memory represented by a flash memory which dominates the market for portable machines, lower power consumption is essential. Since reduction in the operating voltage is strongly required, implementation of a substantially thinner insulating film (tunnel oxide film 104) as thin as that of other ULSI devices is necessary.
As described above, the stress induced current becomes undesirably large if a film thickness of tunnel oxide film 104 is less than 10 nm. Therefore, it is conventionally impossible to make tunnel oxide film 104 thinner. As a result, the operating voltage cannot be lowered, making it difficult to lower power consumption.